The life of a water heater storage vessel tends to be decreased because of an increase in temperature over the normal operating temperatures as the result of the buildup of lime or other sediment in the bottom of the storage vessel. In addition the service efficiency of the vessel is decreased because of the amount of fuel required for the heating unit to perform the heating task. The warning device provided by this invention overcomes these problems because it warns the user of the storage vessel to remove the insulating sediment from the bottom of the heater before a dangerous condition arises.